Recently, there has been increasing interest in energy storage technologies. As the application fields of energy storage technologies is extending to mobile phones, camcorders, notebook computers and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices. In particular, secondary batteries capable of repeatedly charging and discharging have attracted considerable attention as the most promising electrochemical devices. In recent years, extensive research and development have been conducted to design new electrodes and batteries for the purpose of improving capacity density and specific energy of the batteries.
Many secondary batteries are currently available. Lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries such as Ni-MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire or explosion, encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. More research urgently needs to be done to improve the relatively low capacities and insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries.
Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration for safety is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, regulatory guidelines strictly restrict potential dangers (such as fire and smoke emission) of electrochemical devices. Overheating of an electrochemical device may cause thermal runaway or puncture of a separator may pose an increased risk of explosion. In particular, porous polyolefin substrates commonly used as separators for electrochemical devices undergo severe thermal shrinkage at a temperature of 100° C. or higher in view of their material characteristics and production processes including elongation. This thermal shrinkage behavior may cause short circuits between a cathode and an anode.
In order to solve the above safety problems of electrochemical devices, a separator has been suggested in which a mixture of filler particles, such as inorganic particles, and a binder polymer is coated on at least one surface of a highly porous substrate to form a porous coating layer. For example, Korean Unexamined Patent Publication Nos. 2007-83975 and 2007-0019958 and Japanese Patent Publication No. 2005-536857 disclose techniques concerning separators, each of which includes a porous substrate and a porous coating layer formed on the porous substrate wherein the porous coating layer is composed of a mixture of insulating filler particles and a binder polymer and contains a material having a shutdown function.
The insulating porous coating layers formed on the porous substrates contribute to an improvement in the stability of electrochemical devices but inevitably bring about an increase in the thickness of the separators, making it difficult to fabricate high-capacity batteries.